Method of making a coated cemented carbide product

ABSTRACT

A high-strength, coated cemented carbide product comprising a cemented carbide substrate and a fully dense alpha aluminum oxide coating on the substrate. The coating has a thickness of from 120 microns and is firmly and adherently bonded to the cemented carbide substrate through a thin intermediate nonmetallic layer of an iron group metal aluminate. The coated product combines a wear resistance substantially as high as aluminum oxide cutting materials and a transverse rupture strength of at least 150,000 psi. The coated product is prepared by passing water vapor, hydrogen gas and an aluminum halide over the substrate at a temperature of from 900*-1250*C., the ratio of water vapor to hydrogen gas being between about 0.025 and 2.0.

[ Oct. 21, 1975 METHOD OF MAKING A COATED CEMENTED CARBIDE PRODUCT [75]Inventor: Thomas Eugene Hale, Warren,

Mich.

[73] Assignee: General Electric Company,

1 Schenectady, NY.

[22] Filed: Mar. 12, 1973 [21] Appl. No.: 339,653

Related U.S. Application Data [62] Division of Ser. No. 147,240, May 26,1971, Pat. No.

[52] U.S. Cl. 427/255; 30/345 [51] Int. Cl. C23c 11/08 [58] Field ofSearch 117/106 R, 169 R; 29/1827; 423/625 [56] References Cited UNITEDSTATES PATENTS 3,251,337 5/1966 Latta et a1. 117/100 x 3,582,271 6/1971Minagawa et al 423/625 3,736,107 5/1973 Hale 29/182.7

3,836,392 9/1974 Lux et a1 117/169 R OTHER PUBLICATIONS Powell et al.,Vapor Deposition, John Wiley & Sons,

Inc, New York, 1966, pp. 384389.

Primary ExaminerRalph S. Kendall Assistant E.ruminerHarris A. Pitlick [57] ABSTRACT A high-strength, coated cemented carbide product comprisinga cemented carbide substrate and a fully dense alpha aluminum oxidecoating on the substrate. The coating has a thickness of from 1-20microns and is firmly and adherently bonded to the cemented carbidesubstrate through a thin intermediate nonmetallic layer of an iron groupmetal aluminate. The coated product combines a wear resistancesubstantially as high as aluminum oxide cutting materials and atransverse rupture strength of at least 150,000 psi. The coated productis prepared by passing water vapor, hydrogen gas and an aluminum halideover the substrate at a temperature of from 900-1250C., the ratio ofwater vapor to hydrogen gas being between about 0.025 and 2.0.

3 Claims, No Drawings METHOD OF MAKING A COATED CEMENTED CARBIDE PRODUCTThis application is a division of application Ser. No. 147,240, filedMay 26," 1971, and now US. Pat. No. 3,736,107.

BACKGROUND OF THE INVENTION This invention relates to a high-strength,coated cemented carbide product and to a process for its preparation.

Cemented carbides are well known for their unique combination ofhardness, strength and wear resistance and are accordingly extensivelyused for such industrial applications as cutting tools, drawing dies andwear parts. It is known that the wear resistance of cemented carbidesmay be enhanced by the application of a thin coating of a highlywear-resistant material, such as, for example, titanium carbide, andsuch coated cemented carbides are finding increasing commercial utilityfor certain cutting tool and machining applications. However, theincreased wear resistance of such coated products has been at thesacrifice of the strength of the substrate which is substantiallyreduced after coating.

Because of its high hardness, wear resistance and low reactivity with awide variety of metals, aluminum oxide has excellent potential as a toolmaterial, and this potential has to some extent been realized with avariety of aluminum oxide cutting materials that are commerciallyavailable. The principal drawback to the more widespread use of aluminumoxide tools is their low strength which rarely exceeds 100,000 psi,using the standard transverse rupture or bend test. This compares with astrength of from 200,000 to 300,000, or even more, .for cemented carbidecutting tools. The low strengthof aluminum oxide tools limits their useto cutting applications where the tool is not highly stressed, such asin finishing cuts. The low strength of aluminum oxide also precludes theuse of such materials in certain types of insert shapes which encounterhigh stresses when locked in a toolholder.

It is an object of this invention to provide a hard, wear-resistantmaterial which combines the extremely high wear resistance of aluminumoxide with the relativ'ely high strength and hardness of cementedcarbide.

It is an additional object of this invention to improve the wearresistance of cemented carbides without substantially reducing theirstrength. It is still an additional object of this invention to providea process for producing a firmly adherent, nonporous, dense coating ofaluminum oxide on a cemented carbide substrate.

SUMMARY OF THE INVENTION The foregoing and other objects of thisinvention are achieved by the vapor deposition under carefullycontrolled conditions of an alpha aluminum oxide coating of from 1-20microns thickness on a cemented carbide substrate. The product containsa cemented carbide substrate and a fully dense alpha aluminum oxidecoating firmly andadherently bonded to the substrate. In addition, thereis present a very thin, intermediate nonmetallic layer of cobalt-,iron-, or nickel aluminate, which acts to metallurgically bond thecoating to the substrate. The coated product has a wear resistancesubstantially equivalent to aluminum oxide base cutting materials and atransverse rupture strength of at least 150,000, in most cases greaterthan 200,000 pounds/sq. inch. At very high cutting speeds, greater thanabout 1,500 surface ft./minute in some applications, possibly higher inothers, the higher heat resistance of solid aluminum oxide may result inhigher wear resistance. But in all cutting tests other than those abovethese levels, the wear resistance of the present coated products hasproven to be substantially as high as aluminum oxide cutting materialsWhile the broad range of coating thicknesses useful in the invention isfrom l-20 microns, most coating thicknesses are preferably less than 15microns. As will be shown in more detail below, certain applicationsrequire even narrower ranges within these limits, e.g. l-3 microns hasproven optimum for machining high temperature alloys and for millingapplications; 6-12microns has proven optimum for steel machining.

The process of the invention comprises passing an aluminum halide, watervapor and hydrogen gas over the carbide substrate at a temperature offrom 900-1250C., the ratio of water vapor to the hydrogen gas beingmaintained between about 0.025 and 2.0, and preferably between 0.05 and0.20.

There have previously been references in the literature of attempts orsuggestions to coat a variety of substrates with aluminum oxide.However, insofar as is known, the coating of a cemented carbidesubstrate with aluminum oxide to produce a fully-dense and adherentcoating has never previously been disclosed. Nor has the unusualcombination of properties exhibited by the present products beenpreviously attainable in either coated or uncoated cutting toolmaterials. The products of the invention are remarkable in severalrespects. Their strength as compared with comparable known coatedcemented carbide materials is considerably higher and their cuttingperformance is superior in terms of tool life at intermediate and highercutting speeds. The basis for the foregoing statements will becomeapparent from the discussion and test results set forth below.

The term cemented carbide as used herein means one or more transitionalcarbides of a metal of Groups IVb, Vb, and VIb of the Periodic Tablecemented or bonded by one or more matrix metals selected from the groupiron, nickel and cobalt. A typical cemented carbide contains WC in acobalt matrix or TiC in a nickel matrix.

Because of the demanding requirements normally placed upon a cementedcarbide cutting material, the properties of any coating, the manner inwhich it is bonded to the substrate and its effect on substrate strengthare extremely critical. The coating layer must have high integrity interms of density and smoothness porosity or nonuniformity cannot betolerated. The coating must also be firmly and adherently bonded to thecemented carbide substrate to prevent spalling or separation in use. Inaddition, the coating must not reduce the strength of the cementedcarbide substrate significantly. The products of the present inventionhave been extensively tested and have been found to satisfy all of theforegoing requirements. The coatings are uniform and fully dense, theyare firmly bonded to the substrate and the coated composite retains ahigh proportion of its strength, usually greater than of the transverserupture strength of the uncoated substrate. The achievement of thesecharacteristics in the coated product is believed to be quiteunexpected, particularly in view of the substantial strength reductionsknown to result from the addition of wear-resistant coatings to cementedcarbide substrates. The coated materials of the invention also produce asurface finish in machining operations which appears to be fullyequivalent in quality to solid aluminum oxide cutting materials, thelatter being known to produce the best surface finishes.

, DESCRIPTION OF THE PREFERRED EMBODIMENTS The outstanding properties ofthe aluminum oxidecoated product of the invention depend upon carefulcontrol of the process parameters. The process involves the use of agaseous mixture of hydrogen, water, and an aluminum halide such asaluminum trichloride. Carbon monoxide and carbon dioxide may beoptionally added.

The primary overall deposition reaction is:

3H2O 2AlC| AIZOJ 6HCl.

The most important ingredients in the gaseous reaction mixture aretherefore water vapor and aluminum chloride vapor. However, the aluminumchloride vapor can be formed in several ways during the depositionreaction, as for example by heating solid AlCl powder or by passingchlorine gas over aluminum metal. The water vapor is more convenientlyformed by reacting hydrogen with carbon dioxide in the depositionchamber to form carbon monoxide and water vapor by the water gasreaction:

H; C02 CO +H,O

The amount of water vapor formed in this manner depends upon thetemperature and the initial concentrations of hydrogen, carbon dioxide,carbon monoxide and water vapor in the input gas stream. In order toform a good quality coating of desirable thickness in the temperaturerange of 900-1250C., the ratio of water to hydrogen gases present, afterthe water gas reaction, should be between about 0.025 and 2.0.

Hydrogen has been found to be necessary in the vapor deposition processto obtain a dense, adherent coating. Hydrogen appears to insureoxidation of the aluminum at the carbide surface. Oxidation in thereaction zone above the carbide substrate creates a condition known asdusting which must be avoided. The absence of hydrogen creates a porouscoating which is not fully dense. Thus the three necessary ingredientsof the process are aluminum halide vapor, water vapor where a l K; K theequilibrium constant for the water gas reaction; [2 (CO),- (H2O),- K((H2)i+ 2 )i); and 9 at-+0 2),- 2 )i 2 )i 2)i z h l- The parentheses denotethe concentration of the gaseous species enclosed within in terms ofpartial pressure, and the subscripts (b) and (i) denote the final orequilibrium concentrations and the initial or input concentrations,respectively. The amount of H present, and thus the H O/H ratio, canthen be determined from the relationship:

A series of coated products were prepared in accordance with theinvention by passing aluminum chloride vapor, hydrogren and carbondioxide over cemented carbide inserts. The examples were prepared atvarious input gas compositions and at various final H O/Hconcentrations. In all cases, deposition was at l050C. and a -minutedeposition cycle was used with 2-3 grams of aluminum chloride and analuminum chloride generator temperature of about 200C. The use of moreAlCl shifts the desired H O/H ratio to a higher value and vice versa.The coatings were deposited on a cemented carbide substrate having thefollowing composition in by weight: WG-72, Co-8.5, TiC-8, TaC-I 1.5.Table I below shows the effect of gas composition on coating thickness.When coating with both higher and lower ratios of H O/H (i.e. outsiderange of about 0.025 to 2.0), it wasnt possible to get a coating ofsufficient thickness, i.e., 1 micron. Coating quality was good for allexamples having more than 1 micron thickness coating. Coating qualitywas judged to be good if the coating could withstand an adherency testconsisting of sliding the coated insert under a diamond brale indentorof the same type used for the Rockwell hardnes test using a load of 2kilograms on the diamond. If the coating resisted spalling or crumblingduring this test, it was judged to have good quality. If it did not, itwas judged to have poor quality.

TABLE I Water Gas Reaction Equilibrium Coating Input Gas PartialPressures Partial Pressures (H O)+ Thickness Example (H (CO (CO) (H O)(H (H O)+ (1-1 (Microns) and hydrogen. In its preferred form, theprocess includes the use of aluminum chloride vapor, hydrogen and carbondioxide, the latter reacting with H to form water vapor.

The amount of water vapor present, after the reaction of knowninputconcentrations of H and CO and CO and H 0 if used, can becalculated using the following equation:

The nature of the coating obtained was determined by using X-raydiffraction analyses and optical microscopy. X-ray analyses showed thecoating to be alpha A1 0 At the higher deposition temperatures (greaterthan 1150C), significant amounts of the compound W CO C began to formdue to reaction of the substrate carbide with the coating atmosphere.Optical microscopy revealed a gray, translucent coating of Al O that wasfully dense and well bonded to the substrate in those examples in whichthe coating quality was found to be good. A very thin (less than 1micron) layer of another nonmetallic compound, cobalt aluminate (CoAlwaspresent between the A1 0 layer and the ce- 5 mented carbide substrate.The presence of this thin layer is necessary to achieve the proper bondstrength between the coating and the substrate, that is, a bond strengthsufficient to pass the adherency test set forth above. In those cases inwhich no observable intermediate nonmetallic layer was present, thecoated inserts did not pass the above described adherency test. For thisreason, a cobalt (iron, or nickel) aluminate inter- 10 available solidaluminum oxide base (89% A1 0 11% TiO) insert Examples 2022 and a Ti Ccoated cemented carbide insert all run under the same conditions is alsoshown in Table 11.

TABLE II Coating Cutting Time to Transverse Thickness Speed .010 FlankCrater Depth at Rupture Example Insert Type (Mlcrons) SFPM Wear (Min.).010 Flank Wear Strength (PS1) 1 1 A1 0; Coating on Cemented Carbide 1700 9 .003" 260,000 12 4 700 32 .002" 250,000 13 7 700 51 .001" 235,00014 10 700 51 .008 210,000 15 '7 1500 4.2 min. to .0003" (at 235,000 a.004" wear .004 flank wear) 16 A1 0 Coating on Cemented Carbide 7 100017 .007 175,000 17 12 1000 26 .003 160,000 18 Uncoated Carbide 700 4.004" 270,000 19 Uncoated Carbide 1000 5 .010" 230,000 20 Solid A1 0 70051 .001" 90.000 21 1000 .002" 90,000 22 1500 4.5 min. to .0002 (at90,000

.004 wear .004 flank wear) 23 TiC Coating on Cemented Carbide 5 700 18.01 1'' 175,000 24 5 1000 4 .011 175,000

72% WC, 8% TiC, 11.571 TaCv 8.5% Co; 71% WC, 12.5% TiC, 12% TaC, 4.57:Co.

mediate layer is believed necessary to a good quality coating.

The preferred temperature range for deposition of the coating is 900C.to l250C. At lower temperatures, the deposition rate becomes very lowand the coating is poorly bonded to the substrate. At highertemperatures, excessive reaction occurs between the coating atmosphereand the cemented carbide substrate, weakening the bond between thecoating and the substrate and lowering the strength of the overallcomposite body.

The strength of the A1 0 coated cemented carbide composite was measured(as were all strength measurements disclosed herein), using a slightlymodified standard transverse rupture test (ASTM No. B4066-63T), thatincluded three roll loading and a span-to-thickness ratio of 3.5 to 1.Using a deposition temperature of lOC. and a cemented carbide substrateof the nature set forth in the first ten examples in Table I above, theaverage strength of bars having coating thicknesses of from 5-7 microns(the preferred thickness for this substrate in terms of wear resistance)was 241,000 psi. This represents only a slight reduction l 1%) from the270,000 strength value obtained from the uncoated cemented carbidesubstrate.

In the following Table II, the metal cutting performance of coatedinserts prepared in accordance with this invention is shown and comparedwith the corresponding performance of uncoated inserts. Examples 1 1through 17 were /2 X /2 X 3/16 inch disposable cutting inserts, coatedwith A1 0 at 1050C. by the vapor deposition technique disclosed abovefor Examples 1 through 10. A range of coating thicknesses of from 1-l0microns was used. These inserts were then used 3 5 It can be seen thatthe cutting performance of the cemented carbide tool material is verysubstantially improved by the A1 0 coating and that this improvement issubstantially greater than a TiC coating on the same substrate. It isalso evident that the amount of improve- 40 ment obtained is dependentupon coating thickness up to a value of about 7 microns and that someevidence of performance decline occurs at 10 microns. At the optimumthickness value of 7 microns for this substrate, the performance of theA1 0 coated tool was 45 equivalent to that of the solid A1 0 tool at allthree speeds tested. The strength of the A1 0 coated inserts was,however, considerably higher than solid A1 0 and higher than thestrength of the same substrate with a TiC coating.

It should be noted that, because of strength limitations, it has notbeen feasible to use solid aluminum oxide cutting materials indisposable cutting inserts of the type used in pin-type holders. Theseinserts have a centrally disposed hole for the reception of a pin whichlocks the insert in place. The strength of such inserts must besufficient to resist the locking stresses. The strength of the presentcoated materials is sufficiently high to enable their use in suchinserts. The present invention therefore makes possible the use of aninsert, in such applications, having a higher wear resistance than anycomparable insert presently available.

The following Table III shows the performance of the coated inserts ofthe invention irtcutting a high tempersults, Example 25, are comparedwith the perfo gm of an uncoated cemented carbide of the same compo i;

TABLE III ments of the invention, and it is applicants intention in theappended claims to cover all forms which fall within the scope of theinvention.

I claim:

1. A process of coating a cemented carbide substrate with a fully densealpha aluminum oxide coating of from 1 microns thickness comprisingcontacting the carbide substrate with (passing an) Coating ThicknessTime to .020"

Example Insert Type (Microns) Flank Wear (Min.) Comments A1 0 Coating onCemented Carbide 2.5 8.5 26 Uncoated Cemented Carbide 5.4 27 Solid A1 0l. Rapid edge breakdown.

The performance of the insert coated with 2.5 microns of A1 0 wassignificantly better than that of the uncoated cemented carbide insertof the same substrate composition. From tests with other coatingthicknesses, it has been determined that-the optimum thickness for thiskind of machining (i.e., high temperature alloys) is in the 1-3 micronrange. Thicknesses greater than 3 microns in these tests decreased toollife. The superior strength of the A1 0 coated tools is amplydemonstrated by the rapid failure of the solid Al O tool in Example 27,whereas no breakage or chipping was observed in the Al O coated tools,Example 25.

The foregoing is a description of illustrative embodi-

1. A PROCESS OF COATING A CEMENTED CARBIDE SUBSTRATE WITH A FULLY DENSEALPHA ALUMINUM OXIDE COATING OF FROM 1-20 MOCRONS THICKNESS COMPRISINGCONTACTING THE CARBIDE SUBSTRATE WITH (PASSING AN) ALUMINUM HAIDE VAPOR,WATER VAPOR AND HYDROGEN GAS (OVER THE CARBIDE SUBSTRATE) AT ATEMPERATURE OF FROM ABOUT 900* - 1250*C, THE RATIO OF WATER VAPOR TOHYDROGEN GAS BEING BETWEEN ABOUT 0.025 AND 2.0, TO FORM A LAYER OF FULLYDENSE ALPHA ALUMINUM OXIDE ON SAID SUBSTRATE.
 2. The process of claim 1in which the aluminum halide is aluminum chloride.
 3. The process ofclaim 1 in which the water vapor is formed during the coating process bythe reaction of hydrogen and CO2 gas.